vfd output current rating higher than input current rating

I do not work in the AllenBradley field much, but we ran into an incident that I thought was unusual.

Its a 480V vfd. The input current is 59.6.
The output voltage range is 0-460 and the output current rating is 65.

What is the theory behind the output rating being higher than the input? This is the first time I recall seeing that.

Odd that it accepts 480V on the input, but only goes to 460V on the output.

attached is the nameplate.

https://www.allaboutcircuits.com/textbook/direct-current/chpt-2/calculating-electric-power/

You would see a similar phenomenon when comparing input current rating to output current rating in a step down transformer. A VFD is capable of supplying higher output current at a lower voltage without exceeding the drive's power limitation.

Ultimately, the current rating depends upon the capability of the components used on the line side versus the load side of the drive.

Bit_Bucket_07 said:

A VFD is capable of supplying higher output current at a lower voltage without exceeding the drive's power limitation.

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So I sort of assumed that would be true.

Although I also did assume that the drive components are likely rated for x amount of current, rather than x amount of power.

How would I determine what the power limitation is for this vfd? Go by the KW rating on the nameplate?

The voltage rating is dependant on the hz as shown in your picture. The current depends on the input voltage and also the application of the motor....on the nameplate it states up to 112 amperes....which would suggest that the drive settings could go a bit higher....but I have burnt my fingers so many times (customer info) on motor application vs motor size I would try to get more info regarding the application

chud said:

I would try to get more info regarding the application

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its at WWTP. They put in new pumps. The engineers said the vfds are big enough to handle the new pumps. After some issues, the pump manufacturer said, "no, these vfds aren't near big enough"

The pumps are rated at 64.1A @ 480V.

I wouldn't call them big enough either.......Thoughts?

lesmar96 said:

The pumps are rated at 64.1A @ 480V. ......Thoughts?

Click to expand...

64.1A at 480V is 30.8kw


64A at 460V is 29.7kw


Definitely undersized

lesmar96 said:

its at WWTP. They put in new pumps. The engineers said the vfds are big enough to handle the new pumps. After some issues, the pump manufacturer said, "no, these vfds aren't near big enough"

The pumps are rated at 64.1A @ 480V.

I wouldn't call them big enough either.......Thoughts?

Click to expand...

One of the problems with sizing VFDs is that customers want to buy based on hp but the manufacturers want to sell based on amperage.

As a temporary measure, you may be able to limit the max speed to 58 Hz or such, assuming they are centrifugal pumps. Current drops approximately with the square of speed.

Are they centrifugal pumps? These will rarely run at nameplate, unless the duty point is on the extreme right of the curve. Or... sometimes.. when running backwards.

As Tom said, limit the speed slight and see how you go. What are the issues that they've run into? I'd just expect the drive to current limit and not get to commanded speed?

JesperMP said:

Odd that it accepts 480V on the input, but only goes to 460V on the output.

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Is the drive motor parameters configured for 480V?

Saffa said:

Are they centrifugal pumps? These will rarely run at nameplate, unless the duty point is on the extreme right of the curve. Or... sometimes.. when running backwards.

As Tom said, limit the speed slight and see how you go. What are the issues that they've run into? I'd just expect the drive to current limit and not get to commanded speed?

Click to expand...

Exactly. I install and maintain large centrifugal wwts pumps. I've yet to install a vfd on a motor driving such a pump that can actually run full on 60hz. The motor has to be able to drive the mass of the fluid. The higher the flow, the higher the amperage.
Motors are way oversized and are tuned to run at FLA for a given voltage according to the manufacturer datasheet that keeps the pump on curve.

Running at 480/60hz always causes these pumps to cavitate really badly.

JTCat said:

Running at 480/60hz always causes these pumps to cavitate really badly.

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This actually has nothing to do with the VFD or the motor size per se. My bet is that the suction conditions aren't properly considered and the Net Positive Suction Head Available (NPSHA) is less than Required (NPSHR). At higher speeds and flows the head loss on the suction side is excessive and this is causing cavitation. The problem occurs at higher speed but is an indication of improper pump application.

https://www.engineeringtoolbox.com/npsh-net-positive-suction-head-d_634.html

Bit_Bucket_07 said:

A VFD is capable of supplying higher output current at a lower voltage without exceeding the drive's power limitation.

Click to expand...

Good Day Sirs,
I know this thread is several weeks old, but I have a related question.

This WWTP happens to have an Schneider ATV61. It appears to be randomly putting out a very high output voltage.

The incoming power is in the 470s-495. The motor output parameter is set to 460. Most of the time, when its at 60hz, it will put out 460. Very randomly, it will put out as much as 550V.

We left our power quality meter/recorder on for 2 weeks and it happened 3 or 4 times out of those 2 weeks when it was in lead. We should have also been monitoring current, but we didn't have that hooked up.

Is is possible for a VFD to put out more voltage than incoming? My previous understanding is that would not be true?
Does Bit_Bucket's quote above also apply to voltage? That it may increase the voltage as long as its under the power limitation of the vfd? Let's say that somehow the properties of the motor winding were changing and we now needed to produce more voltage to maintain the output torque. Would the drive do that?

In my experience the drive is unlikely to put out higher voltage. If the load becomes overhauling then you may see voltage spike as the motor temporarily functions as a generator.